State-of-the-art electric energy conversion relies on a three-phase power network with alternating currents (AC) at 50 Hz or 60 Hz frequency and a voltage levels ranging from several hundreds of Volts to hundreds of thousands of Volts. The conversion of rotating mechanical energy into electric energy and vice versa is done by generators and by motors, respectively. Those rotating machines can be divided into asynchronous and synchronous apparatuses.
Motors and generators comprise a stator and a rotor. The rotor of the machine rotates inside the stator bore of the stator. Synchronous machines with salient poles generate the magnetic field typically through rotor poles which include a pole core having a rotor winding wrapped around it. The number of rotor poles and the frequency of the stator magnetic field define the number revolutions per minutes (rpm) of the rotating machine. The electric resistance of the winding of a rotor leads to resistive losses therein. In general, these losses need to be considered during design and the rotor needs to be cooled. Cooling mechanisms for rotors typically rely on a cooling fluid such as water, hydrogen or air. This disclosure focuses on air-cooled rotors. The teachings of this disclosure do, however, also apply to other types of machines.
In air-cooled machines, the losses in the form of heat have to be transferred away from the rotor through convection. The effectiveness of cooling through convection depends on flow of air (volume per time), on the temperature of the coolant, and on the coefficient of heat transfer. In certain machines, it can be challenging to supply all regions that need to be cooled with cooling air.
Should a region inside a rotor not be cooled sufficiently, then the machine may locally overheat in that region. Generally speaking, heat transfer through convection is determined by the formulaQ=α·A·ΔT where:    Q denotes the flow of heat per time [W];    α denotes the coefficient of heat transfer [W/m2. K];    A denotes the surface available for cooling [m2]; and    ΔT denotes the temperature difference between solid and fluid temperature [K].
Heat transfer through convection may be influenced by altering the following parameters:    1. The coefficient of heat transfer α depends on the flow characteristics (turbulence) of the coolant fluid and on the characteristics (roughness) of the surface dissipating heat.    2. The temperature difference ΔT is calculated as the difference between the temperature of the surface dissipating heat and the coolant fluid absorbing losses in the form of heat. By lowering the temperature of the coolant fluid, the temperature of the surface dissipating heat will also decrease. Typically, the temperature of the fluid can be lowered by increasing the flux of coolant volume per time. Alternatively, the temperature of the coolant at the inlet may be reduced.    3. Cooling fins may be added to increase the surface A dissipating heat.
Conventional designs make best use of these parameters in an attempt to achieve an optimum result. When the designer runs out of options, he may add additional cooling surfaces to reduce temperatures. This technique is also known as rear ventilation of a rotor coil and is typically applied to salient pole machines. Rear ventilation of a rotor coil means that the rear part of rotor coil forms an active part of the cooling circuit. This measure is, however, seldom applied as it involves a significant design change and especially a major change of the cooling concept of a machine. In addition, rear ventilation of a rotor coil is often in conflict with other mechanical requirements of the machine, especially since additional conduits must be provided for the cooling air. Those additional conduits tend to impair the mechanical integrity of a machine.
Therefore it should be understood that optimizing these parameters can be difficult, because sometimes they are in conflict with other design parameters. Increasing the volume flow for example is having a positive impact on the heat transfer coefficient, but on the other hand the bigger volume flow is creating more ventilation losses. Besides this conflict, it is almost impossible to improve the thermal situation in certain areas of the machine. For example, it is very difficult to increase the amount of cooling air between two pole coil supports because the air path is substantially blocked. If such situation occurs, changing the cooling schema sometimes is an opportunity. One possibility for example is what it is usually known as “back cooling”. In this case the cooling surface is increased by creating an additional air path between rotor winding and pole body. The disadvantages are that there is the danger of dust accumulation in this region (increasing the risk of short circuits) and the weakening of the rotor pole core. The present disclosure is oriented towards providing the aforementioned needs and towards overcoming the aforementioned difficulties.